Alternating current (AC) generators of the Lundell type are well known to those skilled in the art, and include a core of magnetic material supporting a field coil and a pair of end pole segments each having a plurality of teeth that extend axially from the circumference of the segments' base portion, and that interdigitate with the teeth of the other so as to encircle the coil. One such a rotor is disclosed in Gold et al. U.S. Pat. No. 4,588,915, wherein the pole end segments are formed from sheet steel material and the core is formed as a headed steel part.
Another example of a Lundell-type rotor is disclosed in copending U.S. patent application Ser. No. 08/157,452 filed Nov. 22, 1993 in the names of Ward et al. and assigned to the assignee of the present invention, now U.S. Pat. No. 5,382,862. U.S. Ser. No. 08/157,452, now U.S. Pat. No. 5,382,862, discloses a Lundell type AC generator wherein the core and/or the end pole segments are formed from compacted and sintered ferromagnetic particles (i.e., iron). More specifically in Ward et al., the magnetic components of the rotor are formed by coating iron particles having a particle size in a range of about 10 to 250 microns with a thermoplastic material, and then compacting, or pressing, them to the desired shape at a pressure of about 40 to 50 tons per square inch in a heated mold/die. The thermoplastic material acts as a lubricant during the compacting or pressing operation and as a binder to temporarily hold the particles together in a "green" compact. The green compact is subsequently heated at a temperature of about 2050.degree. F. to remove the binder and sinter the particles together into a cohesive mass. The binder can be removed at a lower temperature followed by subsequently heating the compact to the sintering temperature.
Due to their complex shape, compacting of the end segments, as a single piece, often results in end segments which are not homogeneous, in that they have different densities of material at different locations within the segment. Typically, (1) the base (i.e., central) portion of the segment has densities of about 7.3 g/cc, (2) the teeth have densities which vary from about 7.3 g/cc at their roots to about 7.0 g/cc near their tip, and (3) the transition regions (i.e., between the teeth and the base) have densities greater than about 7.4 g/cc as a result of higher pressures in this region and the coincident forging of the particles thereat. Sharp density gradients, as often occur in the transition region between the base and the tooth, can cause cracking in this region, which weakens the teeth and distorts the flux flow. Nonuniform densities are also undesirable in such end pole segment as they can reduce and distort the flux-carrying capacity of the segment primarily at the teeth. On the other hand, if one were to accept some degree of non-uniform density distribution in the end element, it would be better for the teeth to have the higher densities in order to maximize their flux-carrying capacity.
It is an object of the present invention to provide a Lundell-type AC generator wherein the base and teeth of the rotor's pole end segments are formed from iron particles in separate compaction operations, and subsequently joined and sintered into a cohesive mass having an improved density distribution for improved magnetic performance and durability.
This and other objects and advantages of the present invention will become more readily apparent from the description thereof which follows.